Power control during retransmission

ABSTRACT

The present invention relates to a method and a device for controlling power in a network transmitted from a first station to a second station. The second station determines a power target value for a signal received from the first station and sends power control commands to the first station depending on a deviation between said power target value and a received power level. The second station detects faulty data blocks received from the first station and requests retransmission of faulty data blocks from the first station. The adjustment of the power target value to a temporary power target value during the retransmission is performed such that the temporary power target value is calculated depending on the quality of a faulty data block.

FIELD OF THE INVENTION

The present invention relates to a method and a device for controllingpower in a network transmitted from a first station, such as a mobilestation, to a second station, such as a base station.

BACKGROUND OF THE INVENTION

Power control is one of the most important requirements for cellularnetwork systems, such as Universal Mobile Telecommunications Systems(UMTS) employing WCDMA (Wideband Code Division Multiple Access), inparticular on the uplink, i.e. from a mobile station to a base station.

Without a suitable power control mechanism a single mobile station couldblock an entire cell, if that mobile station is overpowered.

Given that two mobile stations are operating within the same frequency,the base station can only separate both mobile stations by theirrespective spreading codes. If the first one of both mobile stations isoperated near by the edge of the cell it may suffer a path loss, e.g. 60dB above that of the second mobile station which is assumed to be nearbythe corresponding base station.

If there would be no power control mechanism for the mobile stations,controlling the respective power levels of both mobile stations, suchthat the base station receives signals from both mobile stations atabout the same level, the mobile station nearby the base station couldeasily “overshout” the mobile station at the edge of the cell. Thus, themobile station nearby the base station could block at least a large partof the cell.

Therefore, it is desirable to equalize the received power per bit of allmobile stations (received by the base station) at any time in order tomaximize network capacity.

A known solution for this kind of problem is the so-called fastclosed-loop power control wherein in the uplink the base station carriesout estimates of the received signal-to-interference ratio (SIR) andcompares the estimated SIR to a target SIR. If the SIR measured for acertain mobile station is higher than the target SIR, the base stationwill send a power control command to the respective mobile stationindicating the mobile station to lower the power. If, however, themeasured SIR is too low, the base station will send a power controlcommand to this mobile station indicating the mobile station to increasethe power.

This mechanism of measuring SIR, sending power control commands, andadjusting the transmission power by the mobile station is performed at arate of 1500 times per second, i.e. with a frequency of 1.5 kHz for eachmobile station. Thus, this mechanism operates faster than anysignificant change of path loss could possibly happen. However, thismechanism applies only to mobile station velocities lower thanapproximately 50 km/h. For velocities higher than this, other featureslike outer loop power control should take over and adjust the EB/N0target to ensure proper operation. Therefore, this power controlmechanism is called fast transmit power control (TPC).

As a result, the base station informs the mobile station bycorresponding power control commands which power level is to be used fortransmission in the next slot (660 μs corresponding to 2560 chips).These power control commands indicate to the mobile station to increaseor decrease the transmission power by a fixed step size, e.g. 1 dB.

Furthermore for enhancing the uplink performance, it has been suggestedin 3GPP (3rd Generation Partnership Project) to use hybrid automaticrepeat control (H-ARQ), wherein the base station in case of a faultyreception of a data block can request a retransmission of the specificdata block.

When requesting the retransmitted data block it is possible to combinethe retransmitted version of the faulty data block with the firstversion of the faulty data block. Thus, the probability of detecting thedata block correctly is increased. In order to achieve a highprobability for correct detection of this data block, the retransmittedversion of the data block is transmitted at the same power level as thefirst version of the respective faulty data block. Thus, there is achance that such a retransmitted data block is finally received with aquality that is higher than the required quality. However, this is awaste of cell capacity.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to improve the cellefficiency.

This object is achieved by a method for controlling power in a networktransmitted from a first station to a second station, wherein saidsecond station determines a power target value for a signal receivedfrom said first station and sends power control commands to said firststation depending on a deviation between said power target value and areceived power level, said second station performing the steps of:detecting faulty data blocks received from said first station,requesting retransmission of faulty data blocks from said first station,and adjusting said power target value to a temporary power target valueduring said retransmission, wherein said temporary power target value iscalculated depending on the quality of said faulty data block.

Furthermore, the above object is achieved by a device for controllingpower in a network transmitted from a first station to said secondstation, comprising: means for determining a power target value for asignal received from said first station, means for generating powercontrol commands for said first station depending on a deviation betweensaid power target value and a received power level, means for detectingfaulty data blocks received from said first station, means forrequesting retransmission of faulty data blocks from said first station,and means for adjusting said power target value to a temporary powertarget value during said retransmission, wherein said temporary powertarget value being calculated depending on the quality of said faultydata block.

Accordingly, the invention utilizes the quality information of thefaulty data block to create a new temporary power target value for useduring the retransmission period of the specific retransmitted datablock. Thus, the retransmitted data block is sent with a transmissionpower level in order to meet a temporary target value defined for thesecond station. Hence, the retransmission power value may be adjusted toa value that is neither too high nor too low, but matches such a valuethat is just needed in order to provide a correct detection of the datablock that has been received faulty during its first transmission. As aresult, the invention improves the transmission power level of the firstterminal device for retransmitted data blocks. Thus, the cell efficiencycan be optimized.

The invention has particularly the following impacts: Interference fromthe first station (the transmitting end) will be lowered as the requiredretransmission power (signal energy) is predicted by the second station(the receiving end) and communicated to the first station by using thepower control commands. Furthermore, the battery consumption of thefirst station will be lowered as only a minimum of additional power isneeded. Finally, it is possible to use a more aggressive data-rateduring the first transmission of a data block as during retransmissionerrors that occurred due to a too aggressive data-rate can be corrected.

Preferably, the quality of the faulty received data block is estimatedas a performance metric indicating how much additional signal energy isrequired in retransmission in order to detect a faulty data blockcorrectly after reception of a retransmitted version of said faulty datablock. Such an estimation is particularly advantageous when the faultydata block is combined with its retransmitted version. Then, the qualityindicates how much additional signal energy is needed and substantiallyonly this required energy is contained in the retransmitted data block.Thus, when combining the faulty received data block with theretransmitted version of this data block, the total signal energy issuch, that the combined signal (first version in addition to theretransmitted version) meets exactly the signal energy that is requiredfor correctly detecting the data block.

Preferably, the temporary power target value for retransmission iscalculated as the power target value for a first transmission of a datablock minus a term derived from the quality, e.g. by multiplying a valueof the quality by a predetermined, in particular fixed, power controlstep size.

Preferably, the adjustment of the power target value is performed at thebeginning of the retransmission of a faulty data block. In order toassure that the received power value is at the target value when thenext data block begins, the power target value transitions from thetemporary power target value to the power target value for firsttransmission of a data block before the next data block begins. Thus,the next data block is received at the desired regular power targetvalue for first transmission of a data block.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greaterdetail based on preferred embodiments with reference to the accompanyingdrawings, in which:

FIG. 1 shows a basic flow diagram of a power control operation accordingto a preferred embodiment of the present invention;

FIG. 2 shows a more detailed flow diagram of the power control operationaccording to FIG. 1;

FIG. 3 shows a basic flow diagram of a block of the diagram according toFIG. 2 depicting the determination of the power target value;

FIG. 4 shows a schematic block diagram of a block shown in the flowdiagram according to FIG. 2 for calculation of the temporary powertarget value;

FIG. 5 shows a timing diagram illustrating the power target value, thetemporary, power target value, the received power values as well aspower control commands; and

FIG. 6 shows a schematic block diagram of a device according to apreferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments will now be described on the basis of anUMTS-WCDMA radio network. However, the invention can be used in otherkinds of networks as well.

FIG. 1 shows a basic flow diagram of the basic operations used in apower control system according to a preferred embodiment of theinvention. These operations are performed at the receiving end of alink, e.g. a base station, in particular a Node B when using a WCDMAUMTS system. The Node B receives a signal from a mobile station, e.g. auser equipment (UE). This link from the UE to the Node B is called theuplink.

However, the invention is not limited to the uplink transmission and canbasically be performed in the downlink transmission as well. In thiscase, the receiving end of the link is the UE and the operationsdepicted in FIG. 1 are performed in the UE.

Generally, Node B informs the UE which power level has to be transmittedin the next time slot, possibly with some latency. Thereby, Node B usespower control commands to inform the UE about the power level which hasto be transmitted in the next slot. These power control commandstypically have fixed step sizes of e.g. 1 dB. However, other step sizesare suitable as well, such as 2.0 dB, 0.5 dB or 0.25 dB.

However, it occurs that the Node B receives faulty data blocks. In thiscase the Node B requests a retransmission of the original data block.The retransmitted data block can be either similar to the first faultyreceived data block or it may contain more redundancy information.

Preferably, when receiving a retransmitted data block, Node B combinesthe newly received information with the previous received information.Thus, the probability of detecting the respective data block correctlyis increased. This so-called hybrid automatic repeat control (H-ARQ) maybecome very efficient in case of usage of aggressive UE transmissionpower, when the UE is usually unable to follow power control“UP”-commands due to hitting maximum power level.

In a first step S 101 Node B detects whether a received data block iserroneous, e.g. by means of cyclic redundancy checks (CRC). If a faultydata block is detected in step S 101 Node B sends in step S 102 arequest for retransmission of the faulty data block to the UE.

Furthermore, Node B performs in step S 103 an estimation of the qualityof the faulty data block. This quality estimation can be based ondifferent mechanisms, e.g. on a bit error rate of the received datastream, a soft information obtained from a Viterby decoder used fordecoding convolutional codes, and/or the signal-to-interference ratio(SIR) of the received signal. Thus, a performance metric is derivedwhich indicates how much additional signal energy is required for theretransmission of a faulty data block in order to detect such faultydata block correctly after receiving its retransmitted version.

Then, in step S 104 this quality information is utilized to create a newtemporary power target value, i.e. the target value for the powerreceived at Node B from UE during the retransmission period of thespecific data block. This temporary power target value is given as EB/N0target for closed loop transmit power control (TPC), wherein EB/N0indicates the level of received bit energy to interference density. Thistarget value indicates the value which is estimated to be needed at NodeB for proper decoding of the signal.

Finally, in step S 105 the power target value is adjusted to thetemporary power target value used during retransmission of the datablock that has previously been received faulty.

FIG. 2 illustrates the power control steps performed in Node B infurther detail. In step S 201 Node B receives a signal or data blockfrom UE. Next, in step S 202 Node B determines the power target value.

FIG. 3 shows in more detail how the power target value is determined. Instep 301 Node B determines whether the received quality is better thanthe required quality. In case of the received quality being better thanthe required quality, it is continued with step S 302 wherein the powertarget value is decreased. In case of the received quality being worsethan the required quality it is continued with step S 303 wherein thepower target value is increased.

Returning back to FIG. 2, it is checked in step S 203 whether a faultydata block is received. In case a faulty data block is detected it iscontinued with step S 204 wherein a message is sent to UE in order torequest a retransmission of the faulty data block.

Furthermore, in step S 205 the quality of the faulty data block isestimated as described above.

Then, in step S 206 a temporary power target value is calculated for theretransmission of the faulty data block. Generally, the temporary powertarget value is calculated as a function of the current power targetvalue and the quality estimate. Thus, a possible mapping into thetemporary target value EB/N0 is given as:Eb/N0_target_retrans=f(EB/N0_target, quality)wherein Eb/N0_target_retrans is the temporary power target value forretransmission and Eb/N0_target is the power target value for firsttransmission of a data block.

FIG. 4 shows a more specific implementation of calculating the temporarypower target value.

On the one hand a value of the quality is weighted by a predeterminedpower control step size 401 that is input to a multiplication unit 402as well as the quality value.

On the other hand the power target value determined in step S 202 isused in step S 206 as follows: Both, the power target value as well asthe result of the multiplication unit 402 are fed to a subtraction unit403 which subtracts the result of the multiplication unit 402 from thepower target value in order to create the temporary power target valuefor retransmission.

As an example it is assumed that Node B has a quality scale where “0” isthe worst quality estimation and “4” indicates a high reception quality.Then, a possible mapping into the temporary target value EB/N0 is givenas:Eb/N0_target_retrans=Eb/N0_target−quality* x dBwherein Eb/N0_target_retrans is said temporary power target value forretransmission, Eb/N0_target is said power target value for firsttransmission of a data block, and x is a fixed power control step sizein dB.

This means that data blocks received with poor quality in the firsttransmission will be received with the same EB/N0 target for theretransmitted version, thus giving approximately 3 dB combining gainfrom H-ARQ, while the almost corrected received data blocks will bereceived at −4 dB EB/N0 target compared with the fist transmission ofthe respective data block. It is noted that Node B combines theinitially faulty received data block with the retransmitted version ofthe data block.

Furthermore, it is noted that the scale given in the above example from0 to 4 could be defined in any other way. However, this mechanismassures that the received power level at Node B is implicitly controlledby Node B through the EB/N0 target and that the power control commandssent to the UE are such that the received signal energy is just as highas required, i.e. the received signal energy is not substantially higheror lower than the required signal energy.

Returning back to FIG. 2, in step S 203 a branch is made to return tostep S 201 if a data block is received correctly.

Substantially at the same time, as step S 203 is performed, a branch ismade from step S 202 to step S 207. Thus, the path starting from S 202is split and two separate flows are initiated. Node B determines in stepS 207 whether a currently received data block is a retransmitted datablock or a data block sent for the first time.

If a currently received data block is a retransmitted version of apreviously sent data block, that has been received faulty, a branch ismade to step S 208 wherein the temporary power target value derived instep S 206 is used for the following power control commands during theretransmission of the faulty received data block. Although steps S 203and step S 207 may be performed substantially at the same time step S208 is performed after step S 206 in order to be able to process thecurrent temporary power target value generated in step S 206. However,it is noted that other sequences of the steps describe herein arepossible as well. Therefore, the invention is not restricted to thisspecific embodiment.

In step S 209 power control commands generated on the basis of thetemporary power target value are sent to UE.

Returning back to step S 207, if the currently received data block is adata block transmitted for the first time the power target value asdetermined in step S 202 is used in step S 210 in order to create acorresponding power control command for the UE which is sent in step S211 to UE.

FIG. 5 is a timing diagram illustrating the instantaneous EB/N0 and theEB/N0 target for H-ARQ. During a first frame 501 comprising a number of(e.g. 15) slots 502, wherein the duration of a frame is e.g. 10 ms andwherein the duration of a slot is 660 μs corresponding to 2560 chips, afaulty data block 503 is received by Node B from UE.

Although the data block is erroneous, the quality is quite good. Thus,the actual received power level 504 is close to the power target value505. Hence, power control commands 506 are alternating between“increase” or “UP” commands and “decrease” or “DOWN” commands. Thus, thereceived power level 504 is close to the power target value 505.

As, however, the received data block is a faulty data block 503, Node Brequests for a retransmission of this data block. However, theretransmission of the specific data block does not necessarily have tobe the subsequent data block, but can be received one or several datablocks after the faulty data block 503. The retransmitted version ofthis specific data block is indicated as 507.

As in the example of FIG. 5, the first data block 503 is receivedfaulty, but the received quality measure at Node B indicates that thedata block will not require much more signal energy in order to bedetected correctly, a relatively low power (temporary power target value512) can be used for retransmission, i.e. for the retransmitted datablock 507.

It is noted that the adjustment of the EB/N0 target, i.e. from the powertarget value 505 to the temporary power target value 512 takes place atthe beginning 508 of the retransmitted data block 507, in order not toinfluence a previous data block 509.

Furthermore, it is noted that the transition 510 from the temporarypower target value 512 back to the original EB/N0 target state, i.e. theoriginal power target value 505 is such that it is assured that thereceived EB/N0 , i.e. the received power level 504 is at the respectivetarget value when the subsequent data block 511 begins.

Thus, the number of slots 502 that the temporary EB/N0 , i.e. thetemporary power target value 512, is in use depends on the followingparameters: The power control step size, the packet size or data blocksize in slots, and the “distance” between the normal EB/N0 target, i.e.the power target value 505 for first transmission of a data block, andthe temporary EB/N0 level, i.e. the temporary power target value 512 forretransmission of a faulty data block.

As shown in FIG. 5, there is a small delay before the temporary powertarget value 512 is met. During this period the UE transmits more powerthan needed. Preferably, this is taken into account when setting thetemporary power target value 512 for retransmission.

As being indicated in FIG. 5, a power control command 506 may consist ofa single bit indicating whether to increase or to decrease atransmission power level of the UE by a fixed power control step size(e.g. 1 dB).

However, the invention is not limited to such power control commands.Moreover, a power control command may comprise a number of bitsindicating whether to increase or to decrease the transmission powerlevel by certain variable power control step size.

Furthermore, a power control command may comprise a number of bitsindicating explicitly an (absolute) value for the transmission powerlevel of the UE.

Thus, the required transmission or retransmission power can be eitherimplicitly or explicitly communicated to the UE by corresponding powercontrol commands. By explicitly communicating the power control commandsto the UE additional new layer 1(L1) control signaling is used in thedownlink from Node B to UE (on top of ACK/NACK information). Suchadditional information is preferably condensed to a very few bits.

By implicitly communicating the power control commands to the UE anyadditional signaling between Node B and UE is avoided.

FIG. 6 shows a schematic block diagram of a device according to anembodiment of the present invention. A Node B 601 comprises a receiver602 receiving a signal 603 from UE (not shown). Receiver 602 isconnected with a unit 604 for detecting faulty data blocks which in turnis connected to a unit 605 for requesting retransmission of faulty datablocks, in particular generating a request message for retransmission,wherein such message will be send to UE.

Furthermore, unit 605 for requesting retransmission is connected to aunit 606 for adjusting the power target value to the temporary powertarget value which has been communicated to unit 606 from a power targetdetermination unit 607 which is connected with unit 606.

Furthermore, the unit 606 for adjustment of the power target value tothe temporary power target value is connected to a generator 608 forpower control commands which in turn is connected to a transmitter 609for sending such power control commands to UE. Furthermore, transmitter609 is connected with the request retransmission unit 605 in order totransmit the corresponding request message for retransmission of afaulty data block to UE.

Generator 608 for power control commands is furthermore connected to adetection unit 610 which is as well connected to receiver 602 in orderto inform the generator about whether an actual received data block is aretransmitted data block or a data block sent for the first time.Depending on this information generator 608 selects either the temporarypower target value received from unit 606 or the normal power targetvalue received via a corresponding connection from power targetdetermination unit 607. Thus, Node B corresponding to FIG. 6 is equippedin order to perform a method as being illustrated in FIG. 1 to 4 inorder to generate signals according to FIG. 5.

It is noted that the present invention is not restricted to thepreferred embodiments described above. In particular the functionalitiesof Node B and UE can be exchanged so that uplink and downlink will beexchanged as well. Moreover, the present invention is not restricted tothe combination of a mobile station and a base station but can be usedbetween any combination of such stations. The preferred embodiments maythus vary within the scope of the attached claims.

1. A method for controlling power in a network transmitted from a firststation to a second station, wherein said second station determines apower target value for a signal received from said first station andsends power control commands to said first station depending on adeviation between said power target value and a received power level,said second station performing the steps of: detecting faulty datablocks received from said first station, requesting retransmission offaulty data blocks from said first station, and adjusting said powertarget value to a temporary power target value during saidretransmission, wherein said temporary power target value forretransmission is calculated depending on, a quality of said faulty datablock, as the power target value for first transmission of a data blockminus the quality weighted by a predetermined power control step size.2. A method according to claim 1, wherein said quality is estimated as aperformance metric, which indicates how much additional signal energy isrequired during retransmission in order to detect a faulty data blockcorrectly after receiving a retransmitted version of said faulty datablock.
 3. A method according to claim 1, wherein said faulty data blockis combined with its retransmitted version.
 4. A method according toclaim 1, wherein said retransmitted version is similar to the firstversion of said faulty data block.
 5. A method according to claim 1,wherein said retransmitted version contains additional redundancy.
 6. Amethod according to claim 1, wherein said temporary power target valuefor retransmission is calculated as a function of the current powertarget value for first transmission of a data block and the quality. 7.A method according to claim 6, wherein said temporary power target valueis calculated based on the following equation:Eb/N0_target_retrans=Eb/N0_target-quality*x dB whereinEb/N0_target_retrans is said temporary power target value forretransmission, Eb/N0_target is said power target value for firsttransmission of a data block, and x is a fixed power control step sizein dB.
 8. A method according to claim 1, wherein said adjustment of saidpower target value is performed at the beginning of a retransmission ofa faulty data block.
 9. A method according to claim 1, wherein atransition back to the power target value for first transmission of adata block is performed before the beginning of the next data block,such that the received power level is at the power target value forfirst transmission when the next data block begins.
 10. A methodaccording to claim 1, wherein a data block is divided into a number ofslots and wherein the number of slots that said temporary power targetvalue is in use depends on said power control step size, the totalnumber of slots within a data block, and the distance between said powertarget value for first transmission and said temporary power targetvalue.
 11. A method according to any one of the proceeding claims,wherein said temporary power target value is calculated depending on adelay before said temporary power target value is met.
 12. A methodaccording to claim 1, wherein said power control commands respectivelycomprise a bit indicating whether to increase or to decrease atransmission power level of said first station by said fixed powercontrol step size.
 13. A method according to claim 1, wherein said powercontrol commands respectively comprise a number of bits indicatingwhether to increase or to decrease said transmission power level as wellas indicating a variable power control step size.
 14. A method accordingto claim 1, wherein said power control commands respectively comprise anumber of bits indicating an explicit value for said transmission powerlevel.
 15. A method according to claim 1, wherein said step of detectingfaulty data blocks comprises a cyclic redundancy check.
 16. A methodaccording to claim 1, wherein said quality is estimated based on a) abit or packet error rate of the received data stream, b) softinformation obtained from a Viterbi decoder used for decodingconvolutional codes, and/or c) the received signal-to-interferenceratio.
 17. A device for controlling power in a network transmitted froma first station to said second station, comprising: means fordetermining a power target value for a signal received from said firststation, means for generating power control commands for said firststation depending on a deviation between said power target value and areceived power level, means for detecting faulty data blocks receivedfrom said first station, means for requesting retransmission of faultydata blocks from said first station, means for adjusting said powertarget value to a temporary power target value during saidretransmission, wherein said temporary power target value beingcalculated depending on a quality of said faulty data block, as thepower target value and means for calculating said temporary power targetvalue for retransmission as the power target value for firsttransmission of a data block minus the quality weighted by apredetermined power control step size.
 18. A device according to claim17, wherein said second station is a base station and said first stationis a mobile station used in a mobile network, in particular in anUMTS/WCDMA network.
 19. A device according to claim 17, comprising meansfor carrying out a method for controlling power in a network transmittedfrom a first station to a second station, wherein said second stationdetermines a power target value for a signal received from said firststation and sends power control commands to said first station dependingon a deviation between said power target value and a received powerlevel, said second station performing the steps of: detecting faultydata blocks received from said first station, requesting retransmissionof faulty data blocks from said first station, and adjusting said powertarget value to a temporary power target value during saidretransmission, wherein said temporary power target value forretransmission is calculated depending on the quality of said faultydata block as the power target value for first transmission of a datablock minus the quality weighted by a predetermined power control stepsize.